Hearing devices are electronic devices in which sound is recorded by a microphone, is processed or amplified, respectively, in a signal processing unit, and is transmitted into the ear canal of a hearing device user over a loudspeaker which is also called receiver. The amplified or processed sounds which are emitted by the receiver are partially recorded by the microphone. In other words, it must be dealt with a closed loop comprising a hearing device with an output signal and an input signal. It must be noted that the path of the sound energy is not limited to acoustic energy, but also comprises, as the case may be, a mechanical transmission from the output to the input, as e.g. over the housing of the hearing device (so-called body sound). Furthermore, one has realized that over a vent, which is actually used for pressure equalization between the inner ear of the hearing device user and the surrounding, or over electrical paths in the hearing device, signal feedback can occur. It has been shown that of all these possible components, the acoustic signal feedback-contributes the largest part.
The mentioned effects can result in a squealing for hearing devices, which squealing is very uncomfortable for the hearing device user and finally renders the hearing device unusable during the occurrence of the squealing. Although there exists the possibility to keep the gain in the hearing device so small that no buildup and therefore no squealing, which is a result of signal feedback, occurs. Therewith, the use of a hearing device is compromised, to be precise in particular for those applications, by which a large hearing loss must be compensated as it occurs for a person who is hard of hearing, because for such patients a comparatively large gain in the hearing device must be adjusted in order to obtain an adequate compensation.
In order that all gain settings, in particular the maximum possible gain setting, for a hearing device can be used in its full extent, it is absolutely necessary to determine the feedback threshold, which means to know the maximum gain setting for a hearing device, for which maximum gain setting there occurs only just no signal feedback.
Methods to determine the feedback threshold in a hearing device are already known. In U.S. Pat. No. 6,134,329, such a method is described with the aid of which the transfer function of the hearing device is estimated from measurements which are made with a hearing device inserted into the ear of a user. Thereby, the overall transfer function is calculated with different gain values without that the closed loop is being opened. Therewith, so-called optimal Wiener filter models are being used. The transfer function in the forward path and the one in the backward path are being calculated together in the following. From the transfer function in the forward path, the possible instable frequencies and the maximum gain settings can be determined in the hearing device. Furthermore, it is also disclosed how the transfer function in the forward path and the one in the backward path can be calculated from the measurements of the overall transfer function. For these measurements, an additional microphone is inserted into the ear canal of the hearing device user, the insertion being done into the hearing canal preferably through the vent.
It is obvious that these known methods ask for a large processing power in order to obtain the desired information. Furthermore, an additional microphone is being used for this variant, which is based on an in-situ measurement, by which the acoustical but also the mechanical characteristics of the overall system is being changed in a disadvantageous manner, such that, as a consequence thereof, errors will occur in the further calculations to determine the feedback threshold.
Furthermore, reference is made to U.S. Pat. No. 6,128,392 from which the use of a hearing device with a compensation filter in its feedback path in the form of a FIR-(Finite Impulse Response) filter is known. Acoustical and mechanical signal feedback shall be compensated, an impulse at the output of the hearing device being applied in order to determine the filter coefficients of the compensation filter. At the input of the hearing device, the impulse response is measured and the values for the coefficients are being determined for the compensation filter therefrom. It is an integrated signal feedback damping which has an influence on the overall transfer function of the hearing device partly in an undesirable manner because signal components of the desired signal are being damped at the same time.
For the sake of completeness, reference is made to a method to determine the signal feedback threshold, which method is applied in practice. The method consists therein that the gain in the hearing device will be increased step by step until signal feedback occurs. As a result, the corresponding value for the amplification, for which only just no signal feedback occurs, corresponds to the signal feedback threshold. This simple method has the great disadvantage that the hearing device user is exposed to high sound levels. Furthermore, the hearing device must produce a high power during the determination of the feedback threshold.
Therefore, it is an object of the present invention to provide a method which does not incorporate the disadvantages mentioned above.